Current methods used to produce micro sized droplets include the generation of droplets from a capillary tip of a coflowing stream, a stream breakup under different shear flows, an extrusion through a polycarbonate membrane, and a breakup via induced surface instability of liquid jets. However, these methods do not allow for the simultaneous control of both the droplet generation speed and size. For size control, the only feasible method available has been to extrude emulsified droplets through microporous membranes or microchannels. The size of the droplets, however, are limited to the diameter of the channels or pores. In addition, the bulk mixing processes tend not to allow for individual control of the composition of the droplet to generate precise multilamellar vesicles or asymmetric vesicles on demand.
Recent attempts have been made to generate droplets in microfluidic devices. These droplet generation devices are based on shearing mechanisms using the T-junction designs, flow focusing designs, interfacial tension driven method using microcapillary arrays, and the utilization of thin film electrode to meter droplets using either DEP, EWOD, or electrostatic forces. Most of these efforts focused on controlling droplet sizes but not the droplet generation rate. These devices have very little control over the pressure gradient for droplet formation and variations in the channels of these devices tend to cause drastic effects in the droplet formation process. As a result, none of these processes enable the fabrication of precise bi-layer vesicles in microfluidic devices.
Applications have emerged that can take advantage of precision generation of tiny droplets. These applications range from programmable biochemical assays to drug delivery particles. The control of droplet size and a tight droplet size distribution are paramount to these applications, such that the sizes of the droplets would determine the amount of drug encapsulation, polymer concentration and the drug release rate of the vesicle. The ability to control droplet generation speed would determine the analysis and production efficiency of the system. In addition, controlled assembly of multi-lamellar vesicles can encapsulate multiple reagents or drugs for broad applications in medicine and biology.
Thus, it would be desirable to provide systems and methods that facilitate greater control in the droplet formation process and, more particularly facilitate sheath flow to control viscous shear and pressure gradient during the process.